1. Field of the Invention
The present invention relates in general to a fluid-filled cylindrical elastic mount adapted primarily to damp input vibrations received in the axial direction, and more particularly to an improved construction of such an elastic mount which is capable of exhibiting desired damping characteristics with high stability, while maintaining sufficient mechanical strength with respect to the input vibrational load.
2. Discussion of the Related Art
As one type of a mounting device interposed between two members in a vibration system to connect these two member in a vibration-damping fashion, there is known a cylindrical elastic mount wherein an inner and an outer sleeve are disposed in a radially spaced relationship and are elastically connected by an elastic body interposed therebetween, primarily for damping or isolating vibrations applied between the two sleeves in their axial direction. Such a cylindrical elastic mount is suitably used in automotive suspension systems, for example, as an upper support, a strut-bar cushion, a tension rod bushing, and a body mount.
Keeping pace with a recent growing demand for motor vehicles of higher grades, there is an increasing requirement for a cylindrical elastic mount having an improved vibration damping or isolating function. One considered approach to satisfy this requirement is to provide the cylindrical elastic mount with a chamber filled with a fluid. The assignee of this application proposed such a fluid-filled cylindrical elastic mount in the form of an upper support for a motor vehicle, as disclosed in Japanese Patent Application No. 1-99012 filed Aug. 24, 1989, which had not been published at the time when the present invention was made (Feb. 14, 1991).
The proposed fluid-filled cylindrical elastic mount includes (i) an inner sleeve having an annular wing portion which extends radially outwardly from an axially intermediate portion thereof, (ii) a pair of intermediate support members which are opposed to each other in an axial direction thereof with a predetermined axial spacing therebetween, each one of the support members having an annular bearing portion which faces the annular wing portion of the inner sleeve in the axial direction, and a cylindrical portion which extends from the outer periphery of the annular bearing portion in the axial direction toward the other support member, (iii) an elastic body elastically connecting the inner sleeve and the pair of intermediate support members, such that the elastic body and the inner sleeve cooperate to define an annular recess which is open radially outwardly through the axial spacing between the cylindrical portions of the intermediate support members and whose bottom is raised by the annular wing portion of the inner sleeve, and (iv) an outer sleeve fitted on the outer circumferential surfaces of the intermediate support members, so as to close the opening of the annular annular recess and thereby form an annular fluid chamber which has an axially middle restricted portion whose radial dimension is reduced by the annular wing portion. When a vibrational load is applied between the inner and outer sleeves of the thus constructed elastic mount in the axial direction, the elastic mount exhibits an excellent vibration damping effect based on the resonance of the fluid in the fluid chamber, that cannot be obtained on an elastic mount which does not have a fluid chamber, i.e., which relies only on the elastic property of an elastic body.
However, a further study by the applicant indicated a problem with the fluid-filled cylindrical elastic mount described above in which the two separate opposed intermediate support members spaced apart from each other in the axial direction are held in position by the outer sleeve which is fitted on the outer circumferential surfaces of the cylindrical portions of the intermediate support members. That is, this cylindrical elastic mount suffers from insufficient mechanical strength of the outer sleeve to withstand an axial force which acts on the outer sleeve and the intermediate support members so as to move the two support members relative to each other in the axial direction.
Explained more specifically, a relative axial displacement of the two intermediate support members away from each other can be prevented by the outer sleeve which is caulked against the intermediate support members, or by suitable stop members attached to a mounting bracket fixed to the outer sleeve. However, it is considerably difficult to prevent a relative axial displacement of the two support members toward each other and thereby prevent undesirable reduction in the axial spacing between the two support members when a relatively large axial vibrational load is applied to the elastic mount. The reduction in the axial spacing of the two support members causes deformation or distortion of the elastic body and the annular fluid chamber, which in turn gives an adverse effect on the vibration damping or isolating properties of the elastic mount. Namely, the proposed fluid-filled cylindrical elastic mount is still unsatisfactory and needs to be improved in terms of its mechanical strength and vibration damping stability.